PLASMODIUM VIVAX is the most prevalent human malarial parasite in several areas of Asia and South-Central America. Phylogenetic studies have shown that P. vivax and other Plasmodium spp. currently found in Southeast Asian macaques are part of a monophyletic group from which P. vivax originated as a human parasite as a result of a host switch. The proposed investigation seeks to understand the evolutionary history of P. vivax together with its known sister taxa in Southeast Asian macaques. Understanding the evolutionary history of macaque malarias is important because some of these parasites can infect humans. Overall, the proposed investigation aims to understand the role of positive selection in the evolution of proteins expressed during erythrocyte invasion in P. vivax and related species. This is a critical step in the parasite life cycle. Several of these proteins are considered targets for malaria vaccines. Spurious evidence of positive selection, however, could result from demographic processes (such as population growth and population structure). Thus we propose to assess the demographic histories of P. vivax and macaque Plasmodium spp., using extensive geographic sampling to understand the evolution of putative adaptive variation. This proposal includes two interrelated aims: Specific aim 1: To investigate the demographic histories of P. vivax and related macaque malarial parasite populations. This investigation will allow us to test the following hypotheses: a) P. vivax originated in Asia as a H. sapiens parasite and has a complex demographic history that includes population expansions outside Asia;b) although host switches have been reported in Plasmodium spp., we hypothesize that within-host-species transmission has been be favored over between-host-species transmission. Thus, we expect to find host-specific lineages in multi-host Plasmodium associated with Southeast Asian macaques. Such lineages will provide a suitable framework to estimate the mutation rates at neutral loci, and by so doing, we can improve our understanding of the recent demographic history of P. vivax. Specific aim 2: To investigate the mode of evolution, extent, and maintenance of genetic diversity in erythrocyte invasion proteins (EIP) in P. vivax and related species. We hypothesize that among the EIP, divergence due to natural selection in the P. vivax branch will be observed in proteins which reflect molecular adaptations of this parasite to Homo after it switched from Cercopithecidae hosts. Second, we will sequence loci encoding EIP proteins in extant populations of P. vivax and related macaque malarial parasites. We will explore their observed polymorphism by using comparative and coalescent simulations approaches. We will test the hypothesis that putative adaptive polymorphisms will be observed in P. vivax EIP, especially at those proteins known to bind the erythrocyte. PUBLIC HEALTH RELEVANCE Although P. vivax is the most prevalent malarial parasite outside Sub-Saharan Africa, there is little information about its genetic diversity. Such information is essential for developing and deploying control measures such as vaccines and drugs. This investigation will also provide information about the distribution, frequency, and host-specificity of macaque malarias. Since these parasites have been found to infect humans in Southeast Asia, understanding their distribution and genetic diversity will be an important first step in assessing the disease risk for humans.